Before a baby can talk, he or she relies on non-verbal cues, especially facial expressions, to communicate. Babies also mirror those cues, and in so doing, discover the emotions the cues are attached to. In a recent study published in the Journal of Basic and Applied Social Psychologyresearchers from the University of Wisconsin scientists evaluated over 100 kids and found that that six and seven-year-old boys who had heavily used pacifiers were worse at mimicking emotions expressed by faces on a video. They also interviewed more than 600 college students and discovered that college-age men whose parents reported they had relied on pacifiers scored lower on tests measuring empathy and the ability to evaluate the moods of others. For girls and young women, the researchers found there was no difference in emotional maturity based on pacifier use.

"Females tend to be more precise both in both expressing and reading emotional cues," lead author Paula Niedenthal, PhD, tells Shine. "We don't exactly know how that occurs. One reason might be that be that society encourages girls to read emotions. They might work harder at it." She adds, "Parents talk to girls about emotional processing more than they do to boys. That's not a revolutionary statement." Since boys aren't expected to be as emotional, parents may not compensate for pacifier use by helping them learn in other ways.

The study was inspired by research on people at the other end of the age spectrum—adults who used Botox. Previous research suggested that people who paralyze their facial muscles with injections of Botulinum toxin as a cosmetic procedure not only express less facial emotion after treatment, but they also feel less emotion. "That work got us thinking about critical periods of emotional development, like infancy," said Niedenthal in a statement. "What if you always had something in your mouth that prevented you from mimicking and resonating with the facial expression of somebody?"

Niedenthal acknowledges that asking parents (and babies) to chuck their pacifiers puts her on shaky ground. "Parents hate to have this discussion." She also says pacifier use while sleeping doesn't harm boys emotionally. "We already know from this work that nighttime pacifier use doesn't make a difference, presumably because that isn't a time when babies are observing and mimicking our facial expressions anyway. It's not learning time."

Niedenthal tells Shine her team is currently doing research on other factors, such as whether babies can express emotions while plugged in to a pacifier and if parents are offering pacifiers more to baby boys. For now, she suggests at least considering limiting daytime use—especially for boys. As with any child-rearing choices, "think about why and when you are using certain strategies. Parents offer the pacifier to their babies, they can't use it on their own. We can ask ourselves when is a good and bad time to use it, just like you might ask the same questions about how much screen time is okay."